This invention relates to improved X-ray dispersive and reflective structures and methods of synthesizing the structures. These synthetic structures are free from the constraints of crystalline symmetries and from the restrictive prior art vapor deposition techniques and materials. The improved dispersive and reflective X-ray structures provide controlled reflectivity and can have improved reflectivity in first and higher orders; or for a particular application an increased first order reflectivity and substantially no intensity from higher order reflections. The improved structures have a substantially constant integral reflection coefficient over a wavelength range of interest, including substantially no fluorescence in the wavelength range. The invention has a wide range of applications for substantially all fields requiring the dispersion and reflection of X-rays, particularly with wavelengths from 9.75 to 120 .ANG., such as crystal analyzers for spectrographic analysis, crystal monochrometers, X-ray laser cavity mirrors, X-ray telescopes and certain X-ray photo lithographic equipment.
Commercial X-ray dispersive structures are formed from crystalline structures such as LiF, metal acid phthalaters (map), pyrolytic graphite and Langmuir-Blodgett (LB) films. These materials have very restrictive lattice spacing constraints. In addition, the LB and map devices have severe environmental limitations and must be operated near room temperature in a dry enviroment. LB devices are not appropriate for very high vacuum applications since under certain conditions they can evolve contaminants. They are also inappropriate for high incident beam energy applications since they can decompose. They have poor mechanical integrity, such as scratch resistance, mechanical breaking strength and resistance to abrasion. Further, all of the prior structures have lower reflectivities than desired.
Numerous attempts to construct both natural and new crystalline analogue materials have been made with the aim of extending the X-ray properties heretofore limited by the availability of natural crystalline materials. One such attempt is compositional modulation by molecular beam epitaxy (MBE) deposition on single crystal substrates. For example, in Dingle et al., U.S. Pat. No. 4,261,771, the fabrication of monolayer semiconductors by one MBE technique is described. These modulated prior art structures are typically called "superlattices." Superlattices are developed on the concept of layers of materials forming homo or hetero epitaxially grown planes or films layers resulting in a one-dimensional periodic potential. Typically, the largest period in these superlattices is on the order of a few hundred Angstroms; however, monatomic layered structures have also been constructed.
The superlattices can be characterized by the format of a number of layer pairs formed by a layer of A (such as GaAs) followed by a layer of B (such as AlAs), etc.; formed on a single crystal synthetic material with good crystalline quality and long range order. The thickness of each layer pair (A and B) is defined as "d" spacing. These structures are not appropriate for most reflective or dispersive structures due to the small electron density contrast between the layers. These structures being essentially single crystals with extra super lattice periodicities also suffer from restrictive d spacing, associated with the constraint that the entire structure be a single crystal.
In addition to the MBE type of superlattices construction techniques, other researchers have developed layered synthetic microstructures (lsm) utilizing other forms of vapor deposition, including diode and magnetron sputtering, reactive gas injection and standard multisource evaporation. The layer dimensions are controlled by shutters or moving the substrates relative to the material sources or with combinations of shutters and relative motion. In the case of multisource evaporation, the required thickness control is achieved by monitoring the X-ray reflectivity of the film in situ as the deposition is being made. The materials reported have been formed from crystalline layers, noncrystalline layers and mixtures thereof; however, generally the efforts so far reported are directed at the synthesis of superlattice-type structures by precisely reproducing the deposition conditions on a periodic reoccurring basis. Some of the structures have graded d spacing through the structure.
These materials can be thought of as synthetic crystals or crystal analogues in which it is defined as crucial that the long range periodicity or repetition of a particular combination of layers be maintained. These structures are both structurally and chemically homogeneous in the x-y plane, and are periodic in the third (z) direction. These construction approaches particularly sputtering, can utilize a greater variety of materials than evaporation. The d spacing in a structure can be graded throughout the structure to provide some reflectivity for a range of X-ray wavelengths, but they do not achieve optimum control of higher order reflections and the deposition precision is not as good as desired. This results in interfaces and layer thicknesses which are not as precise as desired for certain applications. While smoothing of the underlying substrate or layers has been reported as multiple layers are deposited, the minimum smoothness reported has been about 1.4 to 1.8 .ANG.. Also, the minimum reported d spacing for any significant reflectivity has been above 15 .ANG.. One desired goal in producing high efficiency X-ray reflectors is to produce a maximum contrast in electron density across the most precisely defined interface which produces the greatest number of orders of reflection. Further, the smoothness of the layer surfaces must be as precise as possible to minimize scattering caused by the surface variations.
Prior attempts at utilizing these structures for X-ray applications relied on the use of carbide low density layers between high electron density metal layers for making high reflectivity structures for the soft X-ray region. Other elements that have been suggested for the low electron density layers and in some cases utilized are boron and beryllium. Although these elements do provide high electron density contrast, they suffer from a very great disadvantage, that is they all have absorption edges in the soft X-ray range in which these structures are to be employed. This means that these structures have highly nonconstant reflectivity and resolution across the wavelength range of interest. In addition to the variability of the reflecting properties, these elements also have large fluorescences in the wavelength range of interest which degrade the signal to noise ratio especially when wavelengths near the fluorescence wavelengths are utilized.